1. Field of the Invention
The present invention relates to video systems. More particularly, the present invention relates to expanding color samples in a compressed video image.
2. The Background Art
There are many color models used to represent and display color graphics on a computer monitor or color television display. The red, green and blue (RGB) model, or RBG color space, is the most prevalent. With RGB color space the three components are the three primary colors and each corresponds to an intensity that may be used to illuminate the red, green and blue phosphors on a cathode ray tube (CRT). The relationship between each RGB component value and an amount of phosphor illumination makes RGB color space a good choice for video displays. There is a minor complication with RGB color space due to a non-linearity between a video signal voltage and the color intensity displayed on a CRT. The non-linearity is well known to those of ordinary skill in the art and is typically addressed by using "gamma-corrected" RGB values, often denoted R'G'B'. This disclosure will ignore the distinction between RGB and R'G'B' signals and use the term RGB when referring to either.
There are, however, reasons to use color spaces other than RGB for video transfer and storage. First, RGB color space typically requires that all three components have the same number of data bits and resolution. For example, 24 bit video signals, with 8 bits for each of the three RGB components, are commonly used for computer video displays. This can be a disadvantage for some video compression techniques. Second, using video signal processing procedures, such as adjusting the brightness of an image, with RGB color space may require modifying each of the three components separately. Other color space models are more efficient in terms of video signal processing. Third, legacy black-and-white television receivers must be supported. Color television standards, in order to remain compatible with black-and-white receivers, typically added two color information signals for use in conjunction with the black-and-white signal. Standards such as these, while using three components to describe an image, are inherently different from RGB color space.
Modern video systems often use one of three non-RGB color space models, YUV, YIQ and YCbCr. Each uses a luma (or luminance) value, denoted as "Y", to depict the basic black-and-white information in an image. Each model also uses two chroma (or chrominance) values to depict the color information, although they each differ in how the chroma values are defined.
Video compression techniques may take advantage of the separate luma and chroma data fields. For example, chroma data may be sampled at only one-half or one-quarter of the rate used for luma data. Doing so can reduce the size of the video data by one-third or one-half of that with equal sampling. Since human vision tends to be more sensitive to the details represented by the luma data, even a 75% reduction in chroma sampling may not significantly degrade the perceived image quality.
The present invention is directed at an apparatus and method for expanding compressed chroma data. The expanded video format may then be converted to RGB color space for display by techniques well known to those of ordinary skill in the art. The distinctions between the different chroma definitions of YUV, YIQ and YCbCr color space are not particularly important to implementing the present invention and the present invention is not intended to be limited to any particular color space. This disclosure will refer to all such luma and chroma color spaces as YUV, although the data may actually conform to the YIQ or YCbCr formats.
FIG. 1 shows the sequence 10 of converting a compressed YUV format graphic file to a displayed image. The present invention is directed at the "scaling up" process 12. A compressed file 14 is expanded yielding a decompressed file 16, although still in a luma/chroma format, such as YUV. The conversion of the expanded YUV format to a RGB format 18 and the eventual display of the graphic image 20 occur "down stream" of the present invention.